1. Field of the Invention
The present invention relates to a gyrotron having a mode converter in the waveguide.
2. Description of the Related Art
There are various means for heating plasma in a fusion reactor. One of them is electron-cyclotron resonance heating by super high-power millimeterwaves. This plasma heating calls for high power oscillators with a frequency at the band of millimeter waves. The gyrotron is deemed promising as this oscillator.
In a case where an output wave of the gyrotron is practically used to heat core plasma in the fusion reactor, the gyrotron is often separated from the core plasma by considerable distance. It is thus necessary that the output wave mode of the gyrotron is converted into TE.sub.01 mode which is smaller in transmission loss and that the wave thus converted in a TE.sub.01 mode is transmitted to the core plasma through a circular waveguide.
This is the reason why attention has been paid to a system disclosed in a below-cited reference (1) and including a mode converter which is located on the way of waveguide passage in a circular waveguide and which is formed by a circular waveguide provided with periodic perturbations to convert the output wave oscillated under the TE.sub.mn mode into that of the TE.sub.01 mode.
Reference (1): M. Thumm, et al. "In-Waveguide TE.sub.01 -To-Whispering Gallery Mode Conversion Using Periodic Wall Perturbations"
Recently, however, millimeter waves higher in frequency and larger in power are needed. The gyrotron of such a large power type that can meet this need oscillates millimeter waves under that mode which is m &gt;&gt;1, n.about.1 under the TE.sub.mn mode and which is called the whispering gallery mode. It is difficult in this case to convert the output wave of this mode directly into that of the TE.sub.01 mode by using the mode converter disclosed in the reference (1).
In the case of the output wave of this whispering gallery mode, the output wave propagated through the circular waveguide tube is radiated like a beam into free space by the Vlasov launcher and this radiated wave is transmitted while being successively reflected and focused by plural curved mirrors, as disclosed in a below-cited reference (2). A system in which focused electromagnetic wave is entered into and transmitted in a waveguide provided with rows of grooves on the inner face in the circumferential direction thereof has been produced. This is a tube which is called a corrugated waveguide tube.
Reference (2): S.N. Vlasov, et al. "Transformation Of A Whispering Gallery Mode, Propagating In A Circular Waveguide, Into A Beam Of Waves".
In short, the mode converter is formed by the Vlasov launcher and the curved mirrors.
In the case of the waveguide passage formed as described above, however, high processing accuracy is needed in making the curved mirrors used to transmit the electromagnetic wave, the drive mechanism for adjusting optical axes, the corrugated waveguide tube and the like. The waveguide passage thus formed is therefore higher in cost as compared with the one formed by the circular waveguide tube.
In a case where an electron beam collector for collecting electron beam is used together with the output waveguide tube in the gyrotron of the type which oscillates the output wave under the whispering gallery mode, the electron beam collector cannot resist thermal loading when the gyrotron is made to have a larger output. It has therefore been considered that the mode converter disclosed in the reference (2) is housed in this gyrotron, as shown in FIG. 7, to separate the electron beam collector from the output waveguide so as to make it possible to use a larger-sized electron beam collector.
According to this gyrotron, a gyrating electron beam shot from an electron gun enters into and is oscillated in a cavity resonator. Electromagnetic wave thus generated in the resonator is transmitted into a mode converter, which comprises the Vlasov radiator and the curved mirror, through the circular waveguide tube connected to the resonator. This electromagnetic wave is reflected by a reflecting mirror in a direction right-angled relative to the center axis of the cavity resonator and then sent as output electromagnetic wave through an output window. FIG. 7 illustrates a mode converter 6, an electron gun 11, an electron beam 12, a circular waveguide 13, a Vlasov mode converter 14, a mirror 15, a window 16, and a gyrated output wave 17. Reference numeral 18 in FIG. 7 denotes electromagnets for adding magnetic field needed to generate the gyrating electron beam, electromagnets 19 for adding magnetic field needed for oscillation, and a collector 20 for collecting electron beam.
In the case of the gyrotron having the above-described arrangement, however, the mode converter 6 comprising the Vlasov converter 4 and the flat or curved mirror 5 is housed in the gyrotron. This makes the gyrotron complicated in structure and damages the axisymmetry of the gyrotron structure. In addition, reliability is reduced relative to the output wave transmitting axis in the gyrotron.
The electromagnetic wave of the whispering gallery mode is hard to transmit with low loss to an intended position through the conventional waveguide passage. Further, when the electromagnetic wave of the whispering gallery mode is to be converted into that of the TE.sub.01 mode in the conventional gyrotron and to be outputted through the gyrotron, the whole of the gyrotron also becomes complicated.